Deposition techniques based on surface modification and thin film condensation by gas/metal plasmas play an increasing role in coating processes for different applications, in particular for making mechanical, electronic, protective and optical coatings. A main advantage of plasma assisted technology is derived from the possibility of varying the energy and trajectories of ions arriving from a plasma to a work piece surface during surface preparation for thin film deposition and initial and further stages of thin-film growth. Variation of ion energy and trajectories can be achieved by work piece biasing. Another advantage of surface modification of work pieces by the plasma assisted technology is related to the extremely high chemical activity of a plasma. Therefore, in using plasma technology it is possible to achieve such chemical reactions that cannot be obtained using conventional methods.
In the following primarily only physical phenomena associated with plasmas will be considered. In particular, the influence of the energy and the space velocity distribution of plasma ions on different phases of surface preparation of work pieces and thin film growth on work pieces will be considered.
Generally, it can be said that the energy of ions incident to a surface of a body can influence such physical processes as gas adsorption and desorption, sputtering, ion implantation, ion deposition or ion plating, collision induced thermal exchange and collision induced surface and bulk diffusion. In particular, increasing the energy of incident ions has many effects that are analogous to effects caused by elevating the temperature of the work pieces.
In regard of surface preparation for thin film condensation ion caused effects of interest are work piece surface cleaning/sputtering, bulk diffusion, and interface mixing. These phenomena are used for increasing the adhesion of a deposited film to a work piece. The efficiency of these processes depends on parameters of the plasma accommodated in a plasma reactor and the biasing voltage used. The higher plasma density the lower biasing potential is required. In a method of processing work pieces using pulsed plasmas, see W-D. Munz, published European patent application No. 1260603, is, because of the low metal plasma density used, such an exotic method as ion implantation used for increasing the adhesion of deposited films. It requires a high biasing voltage, of about 2 kV, and a presence of second and third ions in metal plasmas. To achieve such ions electric discharges with extreme parameters are required. It is obvious that this method has a very limited practical value. More sophisticated methods of processing work pieces using pulsed plasmas are based on the use of methods of plasma production by an increased metal plasma density.
In the conventional deposition method using magnetron sputtering an increased plasma density is usually achieved by an enhanced plasma confinement using open magnetic traps of cusp or mirror geometry, see the book by D. J. Rose and M. Clark, “Plasmas and Controlled Fusion”, M.I.T. Press and John Wiley & Sons, Inc., New York-London, 1961, Northrop T. G. and Teller E., “Stability of adiabatic motion of charged particles in the earth field”, Phys. Rev., No. 117, pp. 215-225, 1960, published Great Britain patent application No. 2 258 343, U.S. Pat. No. 5,554,519, and published European patent No. 0521045. By the use of plasma confinement it is possible to achieve a magnitude of the biasing current in conventional magnetron sputtering that is approximately 100 times greater than in systems not using any magnetic confinement. Plasma confinement and an increased plasma density allow a reduction of the biasing voltage in a cleaning phase down to 1000-600 V, see S. Kadlec et al., “TiN films grown by reactive magnetron sputtering with enhanced ionization at low discharge pressure”, Vacuum, 41 (7-9) 1990, pp. 2233-2238, the cited Great Britain patent application No. 2258343, U.S. Pat. No. 5,554,519 and European patent No. 0521045. However, the use of only plasma confinement does not allow achieving metal plasma parameters that are necessary for an efficient use of the processes as described by Cheng et al. and Gilmore et al, see the discussion below.
Cheng et al., see Y-T. Cheng et al., “A comparison between high- and low-energy ion mixing at different temperatures”, Nucl. Instrum. Methods Phys. Res. B, 64, 1992, pp. 38-47, showed that ballistic mixing, i.e. primary and secondary knock-on events between ion and atoms in the solid, depends only on bombarding ions and target atoms masses and initial ions energy. Gilmore and Sprague, see C. M. Gilmore and J. A. Sprague, J., “Molecular-dynamics study of film growth with energetic Ag atoms”, Vac. Sci. Technol. A. 10, 1992, pp. 1597-1599, investigated the ion stimulated thermal exchange process. The influence of the energy and thermodynamics of the ions on the ion stimulated thermal exchange was evaluated by studies of their molecular dynamics. It was found, see G. K. Hubler, J. A. Sprague, “Energetic particles in PVD technology: particle-surface interaction process and energy-particle relationship in thin film deposition”, Surface and Coating Technology 81, 1996, pp. 29-35, that for an incident energy of the ions in the range of 1-40 eV, the number of atoms in the film mixed into the work piece varies from units up to more than a hundred. This result was obtained by molecular dynamics simulation of 200-atom of Pt deposited on Cu-film.
The energy of atoms sputtered from magnetron sputtering cathodes is of the magnitude of order of 1 eV. It is obvious that interface mixing by film deposition using preferably metal vapor is more problematic than using plasma deposition because plasma ions can be accelerated up to high energy by work piece biasing. Moreover, the integral intensity of interface mixing is larger the higher is the ionization rate of gas and vapor in the process chamber and the higher is the work piece biasing voltage. In simple words, a higher rate both of gas and metal vapor ionization and of work piece biasing has to have a positive influence on the adhesion between a thin film and a work piece.
Plasma assisted film growth can be described particularly by phenomena occurring in ion plating. Ion plating is a term used for atomistic film deposition processes in which the work piece surface and a growing film are subjected to a flux of energetic particles sufficient to cause changes in the interfacial region or film properties compared to the non-bombardment deposition. This term is used in coating technology in the cases where a thin film is deposited by a neutral vapor or a plasma in a highly ionized atmosphere. Ion plating affects adhesion and film properties. In particular, if energetic ions are involved in the condensation process, they increase the mobility of the already absorbed atoms, increase the reactivity of the condensation process and contribute to the heating of the work piece. They produce conditions similar to those which occur at elevated temperatures. Messier et al., see R. Messier et al., “Revised structure zone model for film physical structure”, J. Vac. Sci. Technol. A 2(2), April-June 1984, pp. 500-503, found that the minimum temperature limit for precipitation of dense smooth coatings by intensive ion bombardment also drops with increasing the biasing voltage.
The density and microstructure of deposited hard material coatings can be influenced both by the biasing voltage, i.e. the energy of the bombarding ions, and the current density resulting from the biasing voltage. The structure of deposited films becomes dense if the biasing current density is increased.
Integrated semiconductor circuits generally include many layers of different materials such as dielectric, semiconductor or conductor materials. For developing these materials and in particular deposition methods for depositing material deeply into narrow holes by magnetron sputtering much effort has been made. A main problem arising in this process is the randomised velocity space distribution of neutrals and ions that are used for filling vias or for depositing material in vias. That results in a non-directional via deposition. Therefore, the filling of vias having an aspect ratio of more than about 5:1 becomes difficult but modern and future technology requires an increase of the aspect ratio up to 8:1, 10:1 and more. The solution of this problem is based on replacing vapor deposition by plasma deposition and transformation of the random space distribution of the velocity of the metal ions used into a directional velocity space distribution by a negative biasing of the work piece, also called a substrate. It is obvious that in order to achieve a higher effect of the biasing, it is necessary to have a high metal vapor ionization degree.
Generally speaking, it can be said that the use of methods and corresponding apparatus for plasma processing of work pieces requires development both of methods and apparatus for plasma production and of methods and apparatus for work piece biasing.
For increasing the metal vapor ionization degree such methods as SIP, “Self Ionized Plasma”, SSS, “Sustained Self Sputtering”, and the method briefly called Multi-Pole Hollow Cathode Target have been developed, see for example U.S. patent application Ser. No. 09/373,097, filed Aug. 12, 1999 for Fu, U.S. patent application Ser. No. 09/414,614, filed Oct. 8, 1999 for Chiang, and U.S. Pat. No. 5,178,739 for Barnes. The methods for plasma generation mentioned above have many disadvantages. The main disadvantages include a very high average target power, costly power supplies, and a complicated target cooling procedure.
In 1998 V. Kouznetsov suggested, see the published International patent application No. WO 98/40532, filed Mar. 11, 1998, assigned to Chemfilt R & D AB, and U.S. Pat. No. 6,296,742, that for generation of dense plasmas high current pulsed discharges in crossed fields can be used. The discharges are made in a magnetron magnetic configuration in such way that simultaneously with the sputtering process the ionization of metal vapor and sputtering and reactive gases is achieved by the same discharge. Any additional RF ionization is not required.
The suggested method received interest from the industry, see U.S. Pat. No. 6,413,382 for W. Wang at al., and from basic science, see V. Kouznetsov et al., “A novel pulsed magnetron sputter technique utilizing very high target power densities”, Surf. Coat. Technol., 1999, 122(2-3), pp. 290-293, K. Macak et al., J. “Ionizing sputter deposition using an extremely high plasma density pulsed magnetron discharge”, Vac. Sci. Technol. A (Parts 1 and 2), 2000, 18(40), pp. 1533-1537, and A. Ehiasarian et al., “Influence of high power densities on the composition of pulsed magnetron plasmas”, Vacuum 65, 2002, pp. 147-154.
Simultaneously with development of different methods and apparatus for gas and metal plasma production, also work is presently carried out to develop methods and apparatus of work piece processing, in particular biasing.
R. Gruen has in U.S. Pat. No. 5,015,493 described pulsed biasing of work pieces in such a way that the work piece is a first electrode of pulsed discharges and the metal vapor source is the second electrode. Both the work piece and the vapor source are connected in a serial way to a pulsed power supply and discharge gap. The metal vapor source can be a magnetron sputtering cathode, an evaporation source or an electric arc. In the case of a sputtering cathode the cathode has a negative potential in relation to the work piece that is the anode of the discharge. In this case only neutral vapor is accumulated on the work piece and the biasing current is an electron current. The disclosed process can also be used in combination with an evaporation source and electric arc. In that case a work piece is the cathode of the discharge and the vapor source, i.e. the evaporator or the arc, is the anode. In this patent only an ion plating process is described. It means that interface mixing, sputtering, diffusion, implantation and other processes are not possible. It is so because of a low metal vapor ionization rate and narrow limits of discharge parameters. Discharge parameters are limited by the power corresponding to the narrow limits of the abnormal glow discharges that are required to balance energy losses. The other weakness of this method is that the same discharge is used for plasma production and ion plating. In the use of this method of work piece biasing it is not possible to vary the voltage and current of the plasma production discharge and the biasing voltage and current independently of each other.
A. Belkind has in the published International patent application No. WO 01/29278 described a method and apparatus for work piece biasing in a multielectrode sputtering system. In particular, a method and apparatus are disclosed for causing ion bombardment of a substrate during sputter deposition of an electrically insulating or conducting material on the substrate when using sputtering methods including dual cathodes or dual anodes. A novel electrical circuit including a transformer having a center-tap is disclosed, permitting a potential to be applied to the substrate that is controllable in relation to the plasma potential, without having to provide an additional power supply. Also disclosed are methods which permit the use of a biasing supply, for either DC or high frequency AC, and which can permit a continuous discharging of the surface of the substrate through bombardment with alternatingly ions and electrons.
I. Hiroshi has in U.S. Pat. No. 6,297,165 described a method for etching and cleaning work pieces in a plasma processing apparatus wherein the plasma is generated in a vacuum chamber. To perform etching of a substrate placed on a substrate electrode a voltage monitoring conductor is provided in the vicinity of the substrate electrode, and high frequency power is supplied to both the substrate electrode and the voltage monitoring conductor. Completion of the etching operation is detected by monitoring a self-biasing potential generated in the voltage monitoring conductor.
Y. Naoki has in published U.S. patent application No. 2002/031617 described apparatus and a method for plasma processing including controlled biasing functions. The processing technique uses a plasma to process the surface of a sample such as semiconductor device. The phases of RF biasing voltages are applied to a substrate electrode and an antenna electrode located opposite each other in an alternating way so that they are controlled to be opposite to each other. Either one of the electrodes is forced to always act as having a ground potential. Therefore, the current flowing across the magnetic field for controlling the plasma is decreased, and the potential distribution difference in the surface of the sample to be processed is reduced, so that charging damages can be suppressed. The energy of ions incident to the sample to be processed can be controlled to perform high precision etching. The plasma potential can also be controlled so that the strength of the ion impact to the inner wall of the chamber can be reduced, thereby reducing the number of particles detached from the inner wall of the processing apparatus, thereby improving the throughput.
B. Terry has in the published International patent application No. WO 01/58223 described a system and method for plasma processing of work pieces. A substrate processing system includes a processing chamber, a substrate holder having a floating electrical potential and positioned in the chamber, a gas source for supplying a process gas to the chamber, at least one ion source located in the chamber, and a power source for energizing the ion source by positively biasing the anode and negatively biasing the cathode in a train of pulses of selectably variable duty cycle and magnitude to maintain a selected average current over time, the bias in each instance being relative to the chamber walls. The ion source ionizes the process gas producing ions for processing a substrate placed on the electrically floating substrate holder in the chamber. The floating substrate is biased in accord with the net electric charge thereon as controlled by the flux of energetic electrons. One embodiment includes two such ion sources. In this case, the power source energizes first and second anodes and the cathodes in a time multiplexed manner, such that only one of the first or second ion sources is energized at any time and interactions between ion sources are eliminated.
An analysis of the processing methods described above and others shows that none thereof is acceptable for processing work pieces located in a dense plasma produced by the methods described in the cited published International patent application No. WO 98/40532 and U.S. Pat. No. 6,296,742. As a result it can be said that the art needs development of methods and apparatus for processing work pieces which are suitable for high current, magnetron sputtering deposition processes. Such processing methods and apparatus have to provide efficient sputtering using a low discharge voltage, for ion implantation, interface mixing, gas adsorption and desorption, deposition, i.e. ion plating, collision induced thermal exchange and collision induced surface and bulk diffusion. In particular, such processes have to result in:
A very good adhesion.
Condensation of extremely smooth and dense films.
Transformation of chaotic ions space velocity distribution into directional.
Prior art related to the present application is also disclosed in the published International patent applications Nos. WO 01/98553 and WO 02/103078 for V. Kouznetsov and assigned to Chemfilt R&D AB.
As has been mentioned above, in 1998 it was suggested to use for generation of dense metal and gas plasma high power pulsed discharges in crossed fields. The discharges are made in a standard balanced magnetron magnetic configuration. Because of the high, pulsed discharge power simultaneously with the sputtering process the ionization of sputtering and reactive gases and metal vapor is achieved by the same discharge. Any additional RF ionization or microwave ionization is not required.
The prior method of magnetically enhanced sputtering includes the following basic steps:
(a) providing a magnetic field in a magnetron configuration at the surface of target, from which material is to be sputtered;
(b) providing a sputtering gas to be ionized in a chamber containing a target, and
(c) applying a negative voltage between anode and cathode. The negative voltage is applied in pulses having such a peak voltage and so that in the application of each pulse, a rapid voltage increase is provided at the leading pulse edge in a manner so that during the rapid voltage increase at the leading pulse edge the gas located at and in the region in which the electrons are trapped by the magnetic field first adopts a glow discharge state, then continues to an arc discharge state, and thereafter to a fully ionized state creating a substantially homogeneous plasma having a high rate of ionizing the sputtering gas. Because of a low duty cycle of the discharge pulses in the sequence, 110.sup.-7-10%, the pulsed power of each discharge can be very high whereas the average power is low. The pulsed power can be of order of megawatt for an average power of the magnitude of order of about one or a few milliwatts. The high, pulsed discharge power results in production of dense plasma blobs containing both gas and metal plasmas.
Another method for gas and metal production is described in the published International patent application WO 02/103078 mentioned above. The main difference of this method from that described above is that the process of plasma production is divided in two parts. First, low ionized metal vapor is produced by a low current discharge. Thereupon, a high current ionizing discharge is initialized. The difference of this method for plasma production is that the plasma contains an increased share of metal plasma and that discharges are made in various kinds of balanced and unbalanced magnetron magnetic configurations.
A high plasma density exists mainly during the time period of each high current pulsed discharge between the discharge electrodes. The discharge electrodes are located in a process chamber and/or are parts of the process chamber. The work piece or pieces are located in the process chamber as well.
It was found that the above described charge separation zone plasma can initially be concentrated inside a magnetron magnetic configuration that is a magnetic trap or is of mirror type, see the cited article by T. G. Northrop and E. Teller, and that in the case of an unbalanced magnetron magnetic configuration the plasma escapes therefrom preferably through the magnetic mirror along directions perpendicular to the cathode surface.
In U.S. Pat. No. 5,492,606 for Stauder et al. a method for self-stabilizing deposition of a stoichiometris compound using reactive sputtering is disclosed in which electric pulses or oscillations of a low frequency are superposed on a negative D.C. voltage and/or an RF-voltage to bias the target. The substrate holder can be biased by separate generators providing a DC voltage or an RF-voltage. In particular, the target can be feed with a negative DC voltage on which pulses having a frequency of 50 Hz are superposed, giving extinction of the plasma for 5 ms.
Various methods of work piece biasing have been disclosed. Thus, Wang et al., “Tribological and optical properties of crystalline and amorphous alumina thin films . . . ”, Surf. and Coat. Techn., Vol. 146-147, pp. 189-194, describes that a pulsed d.c. bias was applied to both the aluminum target and the substrate to maintain a stable deposition process and high deposition rate”. Pulses of 20 kHz were provided to the substrate holder and pulses of 50 KHz to the target. The DC substrate bias was −200, −250, −300 V. In published European patent applications Nos. 1106709 and 1094943 for Applied Materials metal-plasma physical vapor deposition MP-PVD is described. The substrate holder is biased using an RF-voltage appearing in bursts, each burst including a plurality of periods of the RF-voltage having a frequency of 13.56 MHz.